SUMMARY The development of speech production is a complex process that depends on auditory, somatosensory, motor, and phonological (representational) systems in the brain. Impairment in any of these systems could result in errors of speech sound production and have long term consequences not only for speech development but language, literacy, and social-emotional development. Current clinical practice for diagnosis of speech sound disorders (SSDs) and assessment of their severity focuses on the phoneme-level accuracy of a child?s speech production, often based on single word standardized assessment. However, longitudinal studies have shown that measures of phoneme production accuracy alone are insufficient to adequately characterize SSDs. Specifically, children who make persistent ?developmental? speech errors (error types considered to be typical errors in younger children) are more likely to demonstrate a resolution to their SSD compared to those who make more atypical errors, inconsistent errors, or distortion of sounds (termed ?disordered? errors). In this project, we will use multimodal neuroimaging (functional magnetic resonance imaging, structural analysis, and magnetoencephalography) to assess engagement of a priori auditory, somatosensory, speech motor and phonological networks in 5-year-old children with SSDs who make predominately ?developmental? speech errors, children who make predominately ?disordered? speech errors, age- gender-matched typically developing (TD) controls, and a cohort of younger TD controls. We expect that ?developmental? errors will be associated with functional and structural immaturity in brain mechanisms supporting phonological processing. In contrast, ?disordered? errors will be associated with functional and structural abnormalities in phonological processing regions, along with disrupted connectivity between these regions and brain mechanisms for auditory perception, and somatosensory-motor function. The results of these studies will provide a foundation by which to clearly differentiate the brain mechanisms underlying speech sound disorders and provide a foundation on which to build for future intervention studies targeting the specific networks in which deficits are identified.